1. Technical Field of the Invention
The present invention generally relates to a digital signal demodulator and modulator and particularly to such a demodulator and modulator for networks where efficient use of allocated frequency bandwidth is desirable.
2. Background Art
The telecommunications industry has been expanding at an unprecedented growth rate. In particular, the wireless sector, including 3G, wireless local area networks, smart appliances, telephone traffic with digital voice encoding, video conferencing, wide area computer network connectivity, Internet service and Bluetooth devices, has grown far beyond expectations and at a much higher rate than the fixed telecommunications counterpart. The content of the wireless sector is also changing, with more and more data being transmitted, including Internet connectivity and live feeds. And, this wireless phenomenon is not limited to any geographical boundaries, as the growth is occurring around the globe. While the content is expanding with new applications arising for use in wireless frequency bands, the amount of bandwidth allocated for these applications is a generally fixed or at least limited resource.
Wireless networks are employed to facilitate the communication between computers and other electronic devices. Network management is thus a control scheme that tries to efficiently use a given bandwidth and in order to transmit the most information. In all cases, it is desirable to maximize the network traffic capacity in a given bandwidth in the presence of interference and noise.
A variety of schemes exist for efficiently partitioning the network elements into communication channels. Frequency domain multiple access (FDMA) schemes assign each terminal to a separate, non-overlapping frequency band. Time domain multiple access (TDMA) schemes assign each terminal to a separate non-overlapping time slot. Code division multiple access (CDMA) schemes assign each terminal to a separate modulating waveform so that the cross correlation between each terminal is negligible.
Digitally modulated signals like binary phase shift keyed (BPSK) and quadrature phase shift keyed (QPSK) signals are transmitted between the various network nodes. Examples include satellite communications networks where terminals transmit through satellite transponders, terrestrial elements where terminals transmit through repeating towers and indoor local area networks where terminals transmit through central repeating elements. In each instance digital data is exchanged through wireless transmission to central repeating elements.
All of these schemes are inefficient in the sense that given sufficient signal to noise strength or coding redundancy, more communicators could use the allocated bandwidth if provided with means for detecting the excess signal margin and means for demodulating signals in the presence of interference.
In the past, prior art communication systems generally utilized Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods to achieve channel access. FDMA refers to a communication channel wherein a signal's transmission power is concentrated into a single radio frequency band. Interference from adjacent channels is limited by the use of band pass filters. However for each channel being assigned a different frequency, system capacity is limited by the available frequencies and by limitations imposed by channel reuse.
In TDMA systems, a channel consists of a time slot or frame in a periodic train of time intervals over the same frequency, with a given signal's energy confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. The system capacity is limited by the available time slots as well as by limitations imposed by channel reuse, as each channel is assigned a different time slot.
One of the goals of FDMA and TDMA systems is to try and prevent two potentially interfering signals from occupying the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) techniques allow signals to overlap in both time and frequency. CDMA signals share the same frequency spectrum and in the frequency or time domain, the CDMA signals appear to overlap one another. The use of CDMA greatly increases the capacity of the analog TDMA/FDMA systems and with a high bit rate decoder permits superior voice transmissions. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. Finally, the scrambled signal format of CDMA eliminates cross talk and makes it more difficult to eavesdrop or monitor calls therefore providing greater security.
In a CDMA system, each signal is transmitted using spread spectrum techniques. The transmitted informational data stream is impressed upon a much higher rate data stream termed a signature sequence. The bit stream of the signature sequence data is typically binary, and can be generated using a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of the higher bit rate signature signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
In operation, a plurality of spread information signals, such as binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) modulation, modulate a radio frequency (RF) carrier and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. The receiver correlates the composite signal with one of the unique signature sequences, and the corresponding information signal is isolated and despread.
Wireless networks are employed for a variety of communications, for example, connectivity between computers. Digitally modulated signals like binary phase shift keyed (BPSK) and quadrature phase shift keyed (QPSK) signals are transmitted between the various network nodes. Examples include satellite communications networks where terminals transmit through satellite transponders, terrestrial elements where terminals transmit through repeating towers and indoor local area networks where terminals transmit through central repeating elements or hubs. In each instance digital data is exchanged through wireless transmission with some control over the communications.
The computer elements connected to these networks provide a variety of user services. Examples include telephone traffic with digital voice encoding, video conferencing, wide area computer network connectivity, and Internet service. In each case it is desirable to maximize the network traffic capacity in a given bandwidth in the presence of interference and noise.
The prior art schemes are inefficient in the sense that given sufficient signal to noise strength or coding redundancy, more communicators could use the allocated bandwidth if provided with means for detecting the excess signal margin and means for demodulating signals in the presence of interference.
There have been attempts at multi user receivers, and there are numerous articles related to the topic based on theoretical postulations, however these also have general deficiencies. One multi user approach demodulates all user signals in an initial stage and forms an interference replica for each user. In subsequent processing all interference replicas except for the desired signal are subtracted from an input signal received to remove the interference. In the following stage, demodulation is made again about the desired signal by using a signal obtained by the initial stage. As a result, the user signal quality is improved as compared with the initial stage, and it is readily apparent that interference cancellation characteristic is gradually improved by repeating this process several times with a multistage structure. Another receiver employs a serial structure for canceling interference. When performing interference cancellation at each stage, the interference replica is transferred between stages and the interference replica is stored in memory. The deficiency here is that signals must have substantially different amplitudes for successive cancellation to be feasible. Since interfering amplitudes are arbitrary, this circumstance rarely occurs. Moreover, successive interference cancellation tends to distort the residual signal, and at some point, the cancellation process renders the signal of interest unrecoverable.
U.S. Pat. No. 5,596,439 ('439) discloses one form of a transmitted digital modulation that allows it to be subtracted out so that a full duplex link fits on a single channel. This method works for a pair of transmissions on a duplex link, and it is complementary to the method of the present invention described herein. The joint detection method described therein works with the reference signal method, as the reference signal is a known waveform that may be subtracted out. However, the method of the present invention is more general. The method in the referenced '439 patent applies to a pair of signals on a duplex link when one of the signals is transmitted by the terminal applying the method. The method of the present invention applies to any number of interfering digital signals on the same channel provided that sufficient signal to noise margin exists to jointly demodulate all of the signals.
In another technique, multipath signals other than the main path signal are considered to be interference and the multipath signals are not utilized to be combined into the desired signal. Multipath is a separate issue that is handled by the design of the present invention as well as many other methods.
The need for adding more users to the existing infrastructure and within a limited bandwidth are generally recognized. Thus far, the efforts of multi-user systems have been hampered when going from the theoretical models to the working models. The present invention describes a working model. What is needed is a more efficient communications system that provides a communication network that dynamically assigns communicators to allocated channel by considering the interference and noise environment with an objective of maximizing the number of communicators and effective bandwidth of the channel.